This invention relates to a fuel vapor emission control.
JP-A-S63-111277 published by the Japanese Patent Office in 1992 discloses a fuel vapor emission control device for an engine. In this fuel vapor emission control device, fuel vapor generated in a fuel tank when the engine has stopped is first adsorbed by activated carbon in a canister, the fuel adsorbed by the activated carbon is desorbed using a manifold vacuum under a predetermined running condition after the engine starts, and the fuel vapor is led into an intake passage downstream of a throttle valve to be burnt.
In the prior art fuel vapor emission control device, the effect on the air-fuel ratio due to the supply of fuel desorbed from the canister to the engine, is treated as a disturbance of the air-fuel ratio, and this disturbance is absorbed by air-fuel ratio feedback control using an oxygen sensor. In other words, if the air-fuel ratio of the engine deviates from a target air-fuel ratio due to purge, this deviation is reflected in the air-fuel ratio feedback correction coefficient, and the fuel injection amount (fuel injection pulse width) is corrected by the air-fuel ratio feedback correction coefficient.
However, when the purge flowrate is increased, the deviation from the basic value of the air-fuel ratio feedback correction coefficient increases, the suppression of air-fuel ratio disturbances other than purge drops, and this causes an increase of exhaust gas emissions (HC, CO). For example, when the air-fuel ratio is controlled to a target air-fuel ratio by varying the air-fuel ratio feedback correction coefficient up to xc2x125%, if the air-fuel ratio feedback correction coefficient varies by +20% due to purge, the margin of the air-fuel ratio feedback correction coefficient on the positive side which can be used for suppressing air-fuel ratio disturbances drops by up to 5%.
It is therefore an object of this invention to precisely estimate the fuel amount which is desorbed from a canister during purge and the corresponding air-fuel ratio in a short time, and permit a large purge amount by correcting the fuel injection amount and decreasing the air-fuel ratio fluctuation based on the estimated air-fuel ratio fluctuation.
In order to achieve above object, this invention provides a fuel vapor emission control device for an engine, comprising a canister which adsorbs fuel vapor generated in a fuel tank of the engine, a purge passage connecting the canister with an intake passage of the engine, a purge valve which opens and closes the purge passage, and a controller functioning to drive the purge valve so that the purge rate is a target purge rate, compute a fuel injection pulse width so that the air-fuel ratio of the engine is a target air-fuel ratio, compute a fuel amount desorbed from the canister using a canister model, correct the fuel injection pulse width based on the computed desorbed fuel amount so that the air-fuel ratio fluctuation of the engine due to performing purge at the target purge rate is reduced, and drive a fuel injector of the engine at the corrected fuel injection pulse width.
The canister model comprises an equation which computes the fuel amount adsorbed by the canister based on the immediately preceding value of the fuel amount adsorbed by the canister and the immediately preceding value of the fuel amount desorbed from the canister, and an equation which computes the fuel amount desorbed from the canister based on the computed adsorption amount and target purge rate.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.